Early warning system for approaching transportation vehicles

ABSTRACT

A radio signaling system (10) provides advanced warning of an approaching transportation vehicle to passengers within a plurality of predesignated pickup regions, thereby minimizing or eliminating the necessity of spending significant amounts of time at roadside pickup points awaiting the arrival of the vehicle. Radio signaling system (10) includes transmitting system (11) on the vehicle broadcasting a radio-frequency signal modulated with a plurality of low-frequency modulation signals having a code of unique frequencies and sequence for each predesignated pickup region. Radio signaling system (10) also includes receiving system (12) selectively adjustable to provide an alarm output signal only upon receipt of the unique low-frequency modulation code for the predesignated pickup region to which receiving system (12) is proximate.

TECHNICAL FIELD

The present invention relates generally to a radio signaling system.More particularly, the present invention relates to a radio signalingsystem for providing advanced warning of an approaching transportationvehicle to passengers within a plurality of preselected pickup regions.

BACKGROUND ART

Buses have been utilized for years as an economical means of masstransportation, and have recently been gaining in popularity. Moreover,as petrochemical based fueld which power the vast majority of motorvehicles throughout the world become more scarce and costly, the publicis being vigorously urged to accelerate its utilization of masstransportation vehicles such as buses to preserve these limitedresources.

Unfortunately, the safety and convenience of bus passengers has not keptpace with this growth in utilization. Passengers are universallyrequired to trek to the nearest roadside pickup point and spendsignificant amounts of time awaiting the arrival of their bus. Duringperiods of inclement weather and in certain neighborhoods spending timeoutdoors often times can be unhealthy and dangerous. These concerns takeon even further significance when the passengers are childrem waiting tobe picked up by a scool bus at a location some distance from their homesor relatives waiting to meet the school bus returning with children.

Thus, although there has arisen a great need for an inexpensive remotesignaling device which would advise passengers of the imminent arrivalof the bus at their pickup point and thereby permit them to remain in aplace of relative comfort and safety until immediately prior to thearrival of their bus, no device of this nature has heretofore beenavailable.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the invention to furnish an economicalsystem for providing advanced warning of an approaching transportationvehicle.

It is another object of the invention to furnish a system for providingadvanced warning of an approaching transporation vehicle, as above,employing radio signaling and providing such warning to passengerswithin a plurality of predesignated pickup regions.

It is still another object of the invention to furnish a system forproviding advanced warning of an approaching transporation vehicle, asabove, which furnishes a unique combination of low-frequency signals tomodulate a radio-frequency signal for each of the predesignated pickupregions.

These and other objects and advantages of the present invention overexisting prior art forms will become more apparent and fully understoodfrom the following description in conjunction with the accompanyingdrawings.

In general, a radio signaling system for providing advanced warning ofan approaching transporation vehicle to passengers within a plurality ofpredesignated pickup region includes a transmitting system on thevehicle and a receiving system in proximity to one of the predesignatedpickup regions.

The transmitting system generates a low-frequency-modulatedradio-frequency signal and includes a transmitter for generating aradio-frequency signal, a plurality of low-frequency generators forgenerating a plurality of low-frequency modulation signals, and amodulator circuit for modulating the radio-frequency signal with each ofthe plurality of low-frequency modulation signal in a preselectedsequence. The modulation circuit receives the plurality of low-frequencymodulation signals, provides a composite modulation signal includingseriatum pulses of each of the plurality of low-frequency modulationsignals to the transmitter, and controls both the sequence in which eachof the plurality of low-frequency modulation signals is combined and theduration of each pulse.

The receiving system provides an alarm signal only when thetransportation vehicle is approaching the predesignated pickup regionwith which that particular receiving system is associated. The receivingsystem includes a receiver receiving the low-frequency-modulatedradio-frequency signal and providing a demodulated output signal, aplurality of low-frequency decoders receiving the demodulated outputsignal each of which is tuned to the frequency of one of the pluralityof low-frequency generators for providing an output signal upon receiptof that low-frequency signal, a synchronous detector circuit receivingthe output signals from all the plurality of low-frequency decoders andproviding an alarm output signal only upon receipt of the output signalsfrom the plurality of low-frequency decoders in the preselectedsequence, and an alarm circuit receiving the alarm output signal fromthe synchronous detector circuit and providing an advanced warning ofthe approaching vehicle to passengers within the predesignated pickupregion with which the receiving system is associated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary radio signaling transmittingsystem in accordance with the concept of the present invention.

FIG. 2 is an exemplary low-frequency-modulated radio-frequency outputsignal generated by the transmitting system depicted in FIG. 1.

FIG. 3 is a detailed block diagram of the radio signaling transmittingsystem illustrated in FIG. 1.

FIG. 4 is a block diagram of an exemplary radio signaling receivingsystem in accordance with the concept of the present invention.

FIG. 5 is a detailed block diagram of the radio signaling receivingsystem shown in FIG. 4, schematically depicting some elements thereof.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

A radio signaling system, generally referred to by the numeral 10, forproviding advanced warning of an approaching transportation vehicle topassengers within a plurality of predesignated pickup regions broadlyincludes transmitting and receiving systems, respectively generallyindicated by the numerals 11 and 12 in FIGS. 1 and 4. As shown in FIG. 1and in more detail in FIG. 3, transmitting system 11 includes tonegenerators 13, modulator and control circuit 14, and transmitter 15, theoutput signal from which is radiated in free space by any suitableantenna 16.

Tone generators 13 include a plurality of individual tone generators 20,21, 22 and 23 and tone selector switches 24, 25, 26 and 27. Tonegenerators 20, 21, 22, 23 may be conventional low-frequency generatorsproviding a sinusoidal output frequency that is variable within theaudio-frequency spectrum (i.e., from approximately 20 to 20,000 Hz) byadjustment of tone selector switches 24, 25, 26 and 27.

Modulator and control circuit 14 includes analog switch 30, modulator31, tone burst time control 32, and transmitter operation time control33. Analog switch 30 may be any of the readily commercially availableanalog switches, such as Model No. MC14016 manufactured by Motorola,Inc. of Chicago, Illinois, for selectively gating any of a plurality ofinput signals, in the present instance the output signals from tonegenerators 20, 21, 22 and 23, to a single output line, which in thepresent instance is received by modulator 31.

Modulator 31 may be any of the innumerable circuits for modulating alow-frequency signal upon a radio-frequency (hereinafter called RF)carrier. Modulator 31 receives the single output signal from analogswitch 30 and modulates the same upon the RF signal generated bytransmitter 15 as explained below.

Tone burst time control 32 controls the sequence of and rate at whicheach of the output signals from tone generators 20, 21, 22 and 23 areswitched by analog switch 30 to modulator 31. Tone burst time control 32includes modulation-pulse time base 34, counter 35, counter resetinverter 36 and decoder and inverter 37. Modulation pulse time base 34may be an astable multivibrator or clock generating a pulse-train outputsignal having a frequency equal to the shortest operating period withinthe desired low-frequency-modulated RF signal. For reasons detailedhereinafter, a modulation pulse time base frequency of 1 Hz, providingpulses having a 1 second period, have been found suitable for useherein.

The pulse-train output signal from modulation-pulse time base 34 isreceived by counter 35, which may be any counter that is compatible withboth modulation-pulse time base 34 and decoder and inverter 37, andcapable of incrementing to a count sufficiently large to permit thedesired sequencing of the output signals from tone generators 20, 21, 22and 23 by analog switch 30. Where, as explained hereinafter, ten periodsare found to be adequate, counter 35 may be a conventional decadecounter having a binary input and a binary-coded-decimal (hereinafterreferred to as BCD) output that is resettable through logic inverter 36.Decoder and inverter 37 receives the BCD output count from counter 35and converts it to a decimal format having a logic convention compatiblewith analog switch 30, which in turn receives the decimal output fromdecoder and inverter 37.

Transmitter operation time control 33 controls the duty cycle oftransmitter 15, that is the ratio of time an RF signal is broadcast,T_(ON), to the time no RF signal is broadcast, T_(OFF). Transmitteroperation time control 33 includes duty-cycle-variable time base 40, J-Kflip flop 41, relay 42 having normally open contact 43, and resetone-shot 44. Duty-cycle-variable time base 40 may be a free runningastable multivibrator or clock generating a pulse-train output signalhaving a frequency variable by an internal rheostant (not shown) fromapproximately one-fifteenth (1/15) to one-one hundred fiftieth (1/150)Hz, thereby providing output pulses every 15 to 150 seconds.

J-K flip flop 41, relay 42, and reset one shot 44 all may beconventional devices connected in the following manner. The pulse trainoutput signal from duty-cycle-variable time base 40 is received by theinverting preset input of J-K flip flop 41. J-K flip flop 41 has its Jinput terminal connected to ground, its K input terminal connected to avoltage supply V₁ of suitable potential to maintain the K input terminalat a high logic level, and provides its Q output signal to the coil (notshown) of relay 42 and to the reset inputs of both modulation-pulse timebase 34 and, through inverter 36, counter 35. Reset one shot 44 receivesthe final count signal from decoder and inverter 37 and thereupongenerates a single output pulse to the inverting clock input of J-K flipflop 41. Normally open relay contact 43 has one side connected to apower source of suitable voltage V₂, such as a filtered vehicle powersupply, and its opposite side connected to both modulator 31 andtransmitter 15 as described hereinafter.

Transmitter 15 may be any conventional RF transmitter and may include RFoscillator 47, buffer 48 and final RF amplifier 49. RF transmitter 15and all its individual components receive the power signal from relaycontact 43. RF oscillator 47 provides a RF carrier frequency to buffer48, which amplifies the RF carrier frequency signal, and also receivesthe output signal from modulator 31. Final RF amplifier 49 receives theamplified RF carrier frequency from buffer 48, combines the same withthe output signal from modulator 31 that is also received by it, andamplifies the resultant low-frequency-modulated RF signal. The outputsignal from final amplifier 49 is fed to antenna 16 for radiation toreceiving systems 12.

Having delineated the detailed construction of transmitting system 11,its operation now may be explained. This operational description shallassume the existence of a fleet of buses each operating on differentroutes and having a plurality of passenger pickup points along eachroute. Each bus shall contain a transmitting system 11 and eachpassenger who desires one may possess a receiving system 12. Beforesetting forth on its route, each bus operator shall set the particularordered combination of two audio frequencies previously selected touniquely identify that route into tone generators 13 by adjustment oftone selector switches 24 and 25. As each bus progresses along itsroute, the operator would at prescribed locations on the route set theparticular ordered combination of two audio frequencies previouslyselected to uniquely identify the immediately upcoming geographic pickupzones or regions by adjustment of tone selector switches 26 and 27. Thiswould permit transmitter system 11 to broadcast the necessary advancedwarning signal to each receiving system 12 within the route segment thebus is then approaching.

In order to more readily understand the specific operation of theexemplary transmitting system 11 shown in FIGS. 1 and 3, thelow-frequency-modulated RF signal originating therefrom illustrated inFIG. 2 should first be examined. The waveform of FIG. 2 depicts two RFtransmit pulses having duration T_(ON). Each transmit pulse is dividedinto nine equal segments having a duration equal to the shortest desiredoperational interval during transmit pulse. In the present instance, theshortest desired operational interval is that for each period oflow-frequency modulation of the radio frequency carrier, chosen to be 1second. All other operational intervals are made a multiple of thisbasic "tone burst" period, labeled T_(TB). For example, it has beenfound to be desirable to allow 2 seconds after each transmit pulse isbegun to permit transmitter 15 to reach a steady state output, denotedwarm-up period T_(WU).

Operation of transmitting system 11 begins with the generation of apulse signal by duty-cycle-variable time base 40, the high to low logiclevel transition of which presets J-K flip flop 41 whereby its Q outputsignal goes to and maintains a high logic level. This in turnsimultaneously removes counter 35 from reset, enabling it to beginincrementation from a count of zero, enables modulation-pulse time base34, and energizes relay 42 so as to close relay contact 43. With relaycontact 43 closed, power is immediately supplied to both modulator 31and transmitter 15, resulting in the generation and broadcast of the RFcarrier by transmitter 15.

Enabled by J-K flip flop 41, modulation-pulse time base 34 incrementsdecade counter 35 at 1 second intervals. The decoded output signals fromdecoder and inverter 37 causes analog switch 30 to sequentially gate theoutput signals from tone generators 20, 21, 22 and 23 to modulator 31between counts 2 and 3, 4 and 5, 6 and 7, and 8 and 9, respectively. Theresult is that for 2 seconds transmitter 15 broadcasts only its RFcarrier and for every other of the next seven 1 second intervals the RFcarrier is sequentially modulated with the selected audio tone signalsfrom tone generators 20, 21, 22 and 23.

Prior to the time at which counter 35 reaches count 9 the pulse outputfrom duty-cycle-variable time base 40 has gone to a high logic level,permitting J-K flip flop 41 to change the logic level of its Q outputsignal upon receipt of the next clock input pulse. When counter 35reaches count 9, the output signal from decoder and inverter 37 triggersreset one shot 44 which provides a single pulse that clocks J-K flipflop 41, thereby changing its Q output signal to a low logic level. Thisin turn immediately disables modulation-pulse time base 34 so thatcounter 35 is no longer incremented, and de-energizes relay 42, openingrelay contact 43. With relay contact 43 open, power is immediately cutoff from both modulator 31 and transmitter 15, resulting in terminationof the generation and broadcast of the RF carrier by transmitter 15.

Transmitter 15 remains inactive until the next pulse high to low logiclevel transition is generated by duty-cycle-variable time base 40,presetting J-K flip flop 41 and beginning a new operating cycle as justoutlined above. Inasmuch as the time during which an RF signal isbroadcast is fixed through a combination of the pulse period ofmodulation-pulse time base 34 and the maximum count of counter 35,variation of the time between pulses from duty-cycle-variable time base40, which directly controls the time between which successive RF pulsesare broadcast, similarly controls the duty cycle of thelow-frequency-modulated RF signal radiated by transmitter 15.

Referring now to FIGS. 4 and 5, an exemplary receiving system 12 can beseen to include a receiver 50 receiving the radiatedlow-frequency-modulated RF signal through antenna 51, tone decoderscircuit 52, synchronous detector circuit 53, and latch and output alarm54. Receiver 50 may be any conventional receiver for providing thelow-frequency component of the modulated RF signal to tone decoderscircuit 52, although the superheterodyne type receiver has been found tobe convenient for use herewith and provides adequate sensitivity.

Tone decoders circuit 52 includes a like plurality of individual tonedecoders 60, 61, 62 and 63 and tone selector switches 64, 65, 66 and 67as that of tone generators 13. Tone decoders 60, 61, 62 and 63 may beany of the commercially available tone decoders, such as thephase-lock-loop tone decoder manufactured by National SemiconductorCorporation of Santa Clara, California having Model number LM567,capable of selective tuning to low frequencies within the audiofrequency spectrum by adjustment of tone selector switches 64, 65, 66and 67 and providing an output signal upon reception of a signal havingthe frequency to which that tone decoder is tuned.

Synchronous detector circuit 53 insures that alarm 54 is activated onlyupon receipt of the preselected sequence of tones for the region inwhich receiving system 12 is assigned to operate. Synchronous detectorcircuit 53 includes time base 70 and synchronous detector 71, which inturn includes like pluralities of noise suppression circuits 72, 73, 74and 75, and J-K flip flops 76, 77, 78 and 79.

Noise suppression circuits 72, 73, 74 and 75, respectively, receive theoutput signals from tone decoders 60, 61, 62 and 63 and provide asharply defined transition from one to the other logic level compatiblewith J-K flip flops 76, 77, 78 and 79 whenever tone decoders 60, 61, 62and 63 first receive or discontinue receiving the audio tone signal towhich they are tuned. The output signals from noise suppression circuits72, 73, 74 and 75 are respectively received by the inverted clock inputsto J-K flip flops 76, 77, 78 and 79.

The J input of J-K flip flop 76 is connected to power supply V₁, fixingthat input at a high logic level. The K inputs of all J-K flip flops 76,77, 78 and 79 are connected to ground. J-K flip flops 76, 77, 78 and 79are cascaded, the Q output signal from J-K flip flop 76 being receivedby the J input of J-K flip flop 77, the Q output signal from J-K flipflop 77 being received by the J input of J-K flip flop 78, and the Qoutput signal from J-K flip flop 78 being received by the J input of J-Kflip flop 79. Connected in this manner the Q output signal from J-K flipflop 79 will only go to a low logic level when J-K flip flops 76, 77, 78and 79 are sequentially clocked, which can only occur when tone decoders60, 61, 62 and 63 receive the audio tone signals to which the same aretuned and in which order they occur.

Time base 70 may be any conventional monostable multivibrator orone-shot receiving the output signal from noise suppression circuit 72and generating a single output pulse having a duration and logic levelnoted below whenever the output signal from noise suppression circuit 72goes from a low to a high logic level, indicative of the receipt of theaudio tone to which tone decoder 60 is tuned. This output pulse isreceived by the inverting reset terminal of each J-K flip flop 76, 77,78 and 79. The duration of the output pulse from time base 70 should beslightly longer than the time between counts 2 and 9 of counter 35,which would be slightly more than 7 seconds in the example herein.

Latch and output alarm 54 may include a conventional latch 90 forretaining the present Q output signal from J-K flip flop 79, a manualreset pushbutton 80, and any compatible audible alarm 91 such as abuzzer connected to latch 90. In addition to or instead of audible alarm91, a visual alarm may be supplied, such as LED 93 whose cathode isconnected, through pull-up resistor 94, to latch 90 and whose anode isconnected to power supply V₁.

Operation of receiving system 12 is straightforward. Initially, toneselector switches 64, 65, 66 and 67 are adjusted to tune tone decoders60, 61, 62 and 63, respectively, to the preselected tones and in thepredesignated sequence which represents the unique radio signaling alarmcode for the region in which receiving system 12 is to operate. Suchadjustment may either be made once the region of interest is known priorto delivery of receiving system 12 to its user, or may be made by theuser who is given the proper tones and sequence for the region ofinterest.

During operation tone decoders 60, 61, 62 and 63 continually monitor thelow frequency component of the received RF signal from receiver 50. Uponreceipt of the preselected audio tone to which tone decoder 60 is tuned,the output signal from tone decoder 60 causes the output signal fromnoise suppression circuit 72 to go to a high logic level. When theoutput signal from noise suppression circuit 72 goes to a high logiclevel, the output of time base 70 is triggered to go to a high logiclevel, removing from reset and thereby enabling J-K flip flops 76, 77,78 and 79. When the preselected audio tone to which decoder 60 is tunedis no longer received, the output signal from noise suppression circuit72 returns to a low logic level, which transition clocks J-K flip flop76 and sets its Q output to a high logic level. Once the Q output signalfrom J-K flip flop 76 goes to a high logic level, J-K flip flop 77 isenabled. If the next audio tone to be received is that to which tonedecoder 61 is tuned, J-K flip flop 77 will be clocked and J-K flip flop78 enabled in a similar manner. This process would continue until tonedecoder 63 receives the audio tone to which it is tuned ultimatelyresulting in the Q output signal from J-K flip flop 79 going to a lowstate and setting the output signal from latch 90 to the same, whereuponboth audible alarm 91 and LED 93 would be activated.

The output signal from time base 70 returns to the low logic level afterslightly more than 7 seconds, resetting and disabling flip flops 76, 77,78 and 79. If the correct sequence of the other three audio tonesfollowing the one which activates tone decoder 60 are not receivedwithin this time period no alarm output signal may be provided. Thissignificantly reduces the chance of false activation of latch and outputalarm 54.

Once activated audible alarm 91 and LED 93 would remain in thatcondition until latch 90 is manually reset by pushbutton 80 which wouldreturn the output signal from latch 90 to a high logic level and disableaudible alarm 91 and LED 93.

Selection of the particular audio frequencies to be employed with radiosignaling system 10 is a function of the specific application selected.For example, where radio signaling system is to be utilized with aschool bus system in a rural community where the various bus routes andpickup locations are widely separated, a narrower separation betweenfrequencies may be acceptable without providing interference. Inparticular, where the selected audio frequencies have only 200 Hzseparation, transmitter 15 has an RF output power level of approximately5 watts and is tuned to the remote control carrier frequency of 27.255MHz, and a superheterodyne type receiver 50 utilized, a substantiallyinterference-free, effective radio signaling range of several miles isachieved.

Several modifications suitable for incorporation into radio signalingsystem 10 should be noted. First, the particular quantity of tonegenerators may be changed in accordance with the parameters of thespecific application of radio signaling system 10. For example, whenutilized in a densely populated urban area with overlapping bus routes,a greater number of tone generators may be desirable. Conversely in arural area with a few widely scattered bus routes, fewer than four tonegenerators may be adequate.

A second modification that should now be evident is the use ofmodulation waveforms other than the sinusoidal low-frequency waveformsillustrated herewith. Although sinusoidal waveforms where chosen hereinfor their low cost and availability, the spectral density of othermodulation waveforms may be preferable for overcoming otherelectromagnetically noisy environments.

The frequency of modulation-pulse time base 34 has been chosen to be 1Hz herein, principally to allow sufficient time for tone detectors 60,61, 62 and 63 of the phase-lock-loop type to lock-in on the receivedmodulation signal. Of course, where other detection circuits ormodulation waveforms are employed the frequency of modulation-pulse timebase 34 may be suitably varied.

The skilled artisan should recognize that the audio tone frequencies andmodulation technique employed in the preferred embodiment is a type ofmultiple-frequency frequency-shift keying. It should be readily apparentthat the concept of the present invention includes any of the multitudeof other suitable types of modulation, such as amplitude and phase shiftmethods.

It should also be noted that in describing the construction andoperation of the logic elements within radio signaling system 10, theso-called "positive true logic" convention has been adopted. As would beunderstood by a skilled artisan, any other circuits employing a similaror different logic convention could be utilized to implement the desiredfunctions, and when so utilized clearly fall within the scope of thepresent invention.

Inasmuch as the present invention is subject to many variations,modifications and changes in detail, a number of which have beenexpressly stated herein, it is intended that all matter describedthroughout this entire specification or shown in the accompanyingdrawings be interpreted as illustrative and not in a limiting sense. Itshould thus be evident that a device constructed according to theconcept of the present invention, and reasonably equivalent thereto,will accomplish the objects of the present invention and otherwisesubstantially improve the art of providing advanced warning of anapproaching transportation vehicle to passengers within a plurality ofpredesignated pickup regions.

We claim:
 1. A radio signaling system for providing advanced warning ofan approaching transportation vehicle to passengers within a pluralityof predesignated pickup regions, comprising:transmitting system means onthe vehicle for generating a low-frequency-modulated radio-frequencysignal includingtransmitter means for generating a radio-frequencysignal, a plurality of low-frequency generators for generating aplurality of low-frequency modulation signals, and, modulator andcontrol circuit means for modulating said radio-frequency signal witheach said plurality of low-frequency modulation signals in a preselectedsequence, includinganalog switch means for receiving each of saidplurality of low-frequency modulation signals and selectively gating thesame to said transmitter means, modulation-pulse time control means forgenerating an output signal to said analog switch means to control thesequence of and rate at which said low-frequency modulation signals aregated by said analog switch means and generating a cycle-complete outputsignal after all said plurality of low-frequency modulation signals havebeen gated to said transmitter means by said analog switch means, andtransmitter operation time control means for selectively gating power tosaid transmitter means and thereby controlling the duration saidtransmitter means generates said radio-frequency signal, and receivingsaid cycle-complete output signal and generating an output signal forcontrolling said modulation-pulse time control means, said output signalfor controlling said modulation-pulse time control means disabling thesame upon receipt of said cycle-complete output signal and subsequentlyenabling the operation of said modulation-pulse time control means insynchronization with said gating of power to said transmitter means;and, receiving system means in proximity to a predesignated pickupregion for providing an alarm signal only when the transportationvehicle is approaching the predesignated pickup region with which thatparticular receiving system means is associated includingreceiver meansreceiving said low-frequency-modulated radio-frequency signal andproviding a demodulated output signal, a plurality of low-frequencydecoders receiving said demodulated output signal each of which is tunedto the frequency of one of said plurality of low-frequency generatorsfor providing an output signal upon receipt of that low-frequencysignal, synchronous detector means receiving said output signals fromall said plurality of low-frequency decoders and providing an alarmoutput signal only upon receipt of said output signals from saidplurality of low-frequency decoders in said preselected sequence, and,alarm means receiving said alarm output signal from said synchronousdetector means and providing an advanced warning of the approachingvehicle to passengers within the predesignated pickup region with whichsaid receiving system is associated.
 2. A radio signaling system, as setforth in claim 1, wherein said modulator and control circuit meansfurther includes modulator means receiving said gated low-frequencymodulation signals from said analog switch means and providing amodulation signal to said transmitter means, said transmitter operationtime control means gating power to said modulator means.
 3. A radiosignaling system, as set forth in claim 2, wherein said modulation-pulsetime control means includes modulation-pulse clock means for generatinga pulse train output signal, counter means receiving said pulse trainoutput signal from said modulation-pulse clock means, counting thenumber of pulses in the same, and providing an output signal indicativeof the instantaneous count, and decoder means receiving saidinstantaneous count output signal from said counter means and providingdecoded count output signals to said analog switch means and saidtransmitter operation time control means, one of said decoded countoutput signals being said cycle complete output signal.
 4. A radiosignaling system, as set forth in claims 2 or 3, wherein saidtransmitter operation time control means includes transmitter operationclock means for generating a pulse train output signal, flip flop meansfor receiving said pulse train output signal from said transmitteroperation clock means and providing said output signal for controllingsaid modulation-pulse time control means, one-shot means for receivingsaid cycle complete output signal from said modulation-pulse timecontrol means and generating a single pulse to reset said flip flopmeans, and relay means for receiving said output signal for controllingsaid modulation-pulse time control means from said flip flop means andselectively gating power to said modulator means and said transmittermeans.
 5. A radio signaling system, as set forth in claims 1 or 3,wherein said synchronous detector means includes a plurality of cascadedflip flops each receiving said output signal from one of said pluralityof low-frequency decoders, the final said flip flop in said plurality ofcascaded flip flops providing said alarm output signal.
 6. A radiosignaling system, as set forth in claim 5, wherein said synchronousdetector means further includes time base means for receiving saidoutput signal from the first of said plurality of low-frequency decodersalso providing its output signal to the first said flip flop in saidplurality of cascaded flip flops, said time base means generating anoutput pulse to enable all said plurality of cascaded flip flops uponreceipt of said output signal from the first of said plurality oflow-frequency decoders.
 7. A radio signaling system, as set forth inclaim 6, wherein the duration of said output pulse from said time basemeans is slightly larger than the time which said radio-frequency signalis modulated by said plurality of low-frequency modulation signals, saidcascaded flip flops being disabled at all times other than duringreceipt of said output pulse from said time base means.
 8. A radiosignaling system, as set forth in claim 7, wherein said synchronousdetector means further includes a plurality of noise suppressioncircuits each receiving said output signal from one of said plurality oflow-frequency decoders and providing a substantially noise-free outputsignal to one of said plurality of cascaded flip flops for toggling thelogic level of the output signal of that particular said flip flop.
 9. Aradio signaling system, as set forth in claim 5, wherein said alarmmeans includes latch means and means for providing an audible alarm. 10.A radio signaling system, as set forth in claim 5, wherein said alarmmeans includes latch means and means for providing a visual alarm.
 11. Aradio signaling transmitting system for providing advanced warning of anapproaching transportation vehicle to passengers within a plurality ofpredesignated pickup regions, comprising:transmitter means on thevehicle for generating a radio-frequency signal; a plurality oflow-frequency generators for generating a plurality of low-frequencymodulation signals; and, modulator and control circuit means formodulating said radio-frequency signal with each said plurality oflow-frequency modulation signals in a preselected sequence, saidmodulator and control circuit means including,analog switch means forreceiving each of said plurality of low-frequency modulation signals andselectively gating the same to said transmitter means, modulation-pulsetime control means for generating an output signal to said analog switchmeans to control the sequence of and rate at which said low-frequencymodulation signals are gated by said analog switch means and generatinga cycle-complete output signal after all said plurality of low-frequencymodulation signals have been gated to said transmitter means by saidanalog switch means, and transmitter-operation time control means forselectively gating power to said transmitter means and therebycontrolling the duration said transmitter means generates saidradio-frequency signal, and receiving said cycle-complete output signaland generating an output signal for controlling said modulation-pulsetime control means, said output signal for controlling saidmodulation-pulse time control means disabling the same upon receipt ofsaid cycle-complete output signal and subsequently enabling theoperation of said modulation-pulse time control means in synchronizationwith said gating of power to said transmitter means.
 12. A radiosignaling transmitting system, as set forth in claim 11, wherein saidmodulator and control circuit means further includes modulator meansreceiving said gated low-frequency modulation signals from said analogswitch means and providing a modulation signal to said transmittermeans, said transmitter operation time control means gating power tosaid modulator means.
 13. A radio signaling transmitting system, as setforth in claim 12, wherein said modulation-pulse time control meansincludes modulation-pulse clock means for generating a pulse trainoutput signal, counter means receiving said pulse train output signalfrom said modulation-pulse clock means, counting the number of pulses inthe same, and providing an output signal indicative of the instantaneouscount, and decoder means receiving said instantaneous count outputsignal from said counter means and providing decoded count outputsignals to said analog switch means and said transmitter operation timecontrol means, one of said decoded count output signals being said cyclecomplete output signal.
 14. A radio signaling transmitting system, asset forth in claims 12 or 13, wherein said transmitter operation timecontrol means includes transmitter operation clock means for generatinga pulse train output signal, flip flop means for receiving said pulsetrain output signal from said transmitter operation clock means andproviding said output signal for controlling said modulation-pulse timecontrol means, one-shot means for receiving said cycle-complete outputsignal from said modulation-pulse time control means and generating asingle pulse to reset said flip flop means, and relay means forreceiving said output signal for controlling said modulation-pulse timecontrol means from said flip flop means and selectively gating power tosaid modulator means and said transmitter means.
 15. A radio signalingreceiving system for use with a transmitting system broadcasting aradio-frequency signal modulated by a plurality of low-frequencymodulation signals for providing advanced warning of an approachingtransportation vehicle to passengers within a plurality of predesignatedpickup regions, the plurality of low-frequency modulation signals havingunique frequencies and sequences for each pickup region,comprising:receiver means receiving the low-frequency modulatedradio-frequency signal and providing a demodulated output signal, aplurality of low-frequency decoders receiving said demodulated outputsignal each of which is tuned to one of the frequencies assigned to thepickup region in which the receiving system is to operate for providingan output signal upon receipt of that low-frequency signal, synchronousdetector means receiving said output signals from all said plurality oflow-frequency decoders and providing an alarm output signal only uponreceipt of said output signals from said plurality of low-frequencydecoders in the unique sequence assigned to the pickup region in whichthe receiving system is to operate, and, alarm means receiving saidalarm output signal from said synchronous detector means and providingan advanced warning of the approaching vehicle to passengers within thepredesignated pickup region with which the receiving system isassociated.
 16. A radio signaling receiving system, as set forth inclaim 15, wherein said synchronous detector means includes a pluralityof cascaded flip flops each receiving said output signal from one ofsaid plurality of low-frequency decoders, the final said flip flop insaid plurality of cascaded flip flops providing said alarm outputsignal.
 17. A radio signaling receiving system, as set forth in claim16, wherein said synchronous detector means further includes time basemeans for receiving said output signal from the first of said pluralityof low-frequency decoders also providing its output signal to the firstsaid flip flop in said plurality of cascaded flip flops, said time basemeans generating an output pulse to enable all said plurality ofcascaded flip flops upon receipt of said output signal from the first ofsaid plurality of low-frequency decoders.
 18. A radio signaling system,as set forth in claim 17, wherein the duration of said output pulse fromsaid time base means is slightly larger than the time which saidradio-frequency signal is modulated by said plurality of low-frequencymodulation signals, said cascaded flip flops being disabled at all timesother than during receipt of said output pulse from said time basemeans.
 19. A radio signaling receiving system, as set forth in claim 18,wherein said synchronous detector means further includes a plurality ofnoise suppression circuits each receiving said output signal from one ofsaid plurality of low-frequency decoders and providing a substantiallynoise-free output signal to one of said plurality of cascaded flip flopsfor toggling the logic level of the output signal of that particularsaid flip flop.
 20. A radio signaling receiving system, as set forth inclaim 16, wherein said alarm means includes latch means and means forproviding an audible alarm.
 21. A radio signaling receiving system, asset forth in claim 16, wherein said alarm means includes latch means andmeans for providing a visual alarm.